I have both 745 Li and M3 Convertible. Since 645 Ci Cabriolet uses the same engine as that of 745 Li, I guess what is with the latter might be with the former as well.
1. Engine and exhaust:
645 Ci Cabriolet -- very very quiet so that you probably aren't intened to drive it faster.
M3 Convertible -- excitingly noisy so that you would like to drive it like a racing car.
(If you like to enjoy both driving fun and sound effects, you better keep your M3)
2. rear seats -- 645 Ci Cabriolet is obviously more spacious than M3 (If you ALWAYS need more spacious rear seats, you better consider 645)
3.M3 is mcuh smaller than 645 -- this keeps M3 more agile when you drive it on a winding road, but if you ALWAYS drive on a, say, two-mile straight road, which is not rare in the States but rare in my country, Taiwan, you better choose the more comfotable 645.
4.Acceleration -- 0 -100km : M3 5.5 sec, 645 6.5 sec. This difference matters to me, but may not to you because I regularly drive M3 with my M3 club members, who all drive very fast.